In a packet exchange apparatus employing an optical switch in an exchange stage and also employing an electrical/optical converter and an optical/electrical converter in an input/output unit, the optical/electrical converter is usually of an ac coupling circuit configuration. In this case, there is presented a problem that, if there is any time period during which the 1 or 0 level continues for a longer time than a pre-set time period or during which there is no packet to be received, data dropout occurs in the packet or in the directly following packet.
As a method for overcoming this problem, there is proposed in, for example, JP Patent Kokai JP-A-4-72939 a packet exchange apparatus employing an optical switch in which a dummy data appending circuit is provided on the transmission side so that dummy data is transmitted during a domain (length of time) in which there is no transmission data while the appended dummy data is removed by a dummy data removing circuit provided on the reception side, so that it is attempted to eliminate occurrence of no time devoid of transmission data on the transmission route there is no continuation of 0 or 1 for a longer time than a preset time, thus eliminating the malfunction of data dropout in the optical/electrical converter. This method consists in sending a dummy packet in the absence of packets for transmission on the input side.
This dummy packet is a pattern which is made up of at least one bit of 0 and the same number of bit of 1, with 0 or 1 not continuing for longer than a pre-set time, and to which is appended a particular code by which the packet can be judged to be a dummy packet. An output unit includes a dummy packet eliminating circuit which discriminates a dummy packet based on the possible presence of the particular code specifying a dummy packet to eliminate the discriminated dummy packet in order to receive only the necessary packets.
In case where, in this configuration, there are packets to be transmitted to the same output unit on plural input units, a contention resolution circuit is used in order to inhibit packet interference due to arrival of plural packets at the same output unit, which would lead to reception of erroneous data.
If a connection request is received from the input unit before the input unit sends a transmission packet and connection requests are made to the sole output unit from plural input units, the contention resolution circuit selects a sole input unit from the plural input units to give the input unit a permission for connection. The remaining input units are inhibited from connection. Since the input units inhibited as to connection are equivalent to the input units devoid of the packets for transmission, these input units send dummy packets. The contention resolution circuit has to detect an input unit devoid of the packets for transmission and an output unit devoid of packets for reception and to find out a connection pattern to interconnect these input and output units.
FIG. 37 shows a configuration of a conventional optical switch used in an exchange stage. Referring to FIG. 37, this optical switch is a 4-input 4-output splitter/combiner type optical switch having four light splitters 10, 16 optical gates 11 and four light combiners 12. For controlling this switch, 42 or 16 control lines are required to control the 16 optical gates.